The document is a great summary of what Web accessibility is and why it is important. It also contains links to further reading, accessibility guidelines, examples, and much more.

Here is one of the whys:

It is essential that the Web be accessible in order to provide equal access and equal opportunity to people with diverse abilities. Indeed, the UN Convention on the Rights of Persons with Disabilities recognizes access to information and communications technologies, including the Web, as a basic human right.